68Ga-Prostate-specific membrane antigen (PSMA) positron emission tomography (pet) in prostate cancer: a systematic review and meta-analysis

ABSTRACT Introduction: Prostate cancer (PC) is the second most commonly diagnosed cancer in males. 68Ga-PSMA PET/CT, a non-invasive diagnostic tool to evaluate PC with prostate-specific membrane antigen (PSMA) expression, has emerged as a more accurate alternative to assess disease staging. We aimed to identify predictors of positive 68Ga-PSMA PET and the accuracy of this technique. Materials and methods: Diagnostic accuracy cross-sectional study with prospective and retrospective approaches. We performed a comprehensive literature search on PubMed, Cochrane Library, and Embase database in search of studies including PC patients submitted to radical prostatectomy or radiotherapy with curative intent and presented biochemical recurrence following ASTRO 1996 criteria. A total of 35 studies involving 3910 patients submitted to 68-Ga-PSMA PET were included and independently assessed by two authors: 8 studies on diagnosis, four on staging, and 23 studies on restaging purposes. The significance level was α=0.05. Results: pooled sensitivity and specificity were 0.90 (0.86-0.93) and 0.90 (0.82-0.96), respectively, for diagnostic purposes; as for staging, pooled sensitivity and specificity were 0.93 (0.86-0.98) and 0.96 (0.92-0.99), respectively. In the restaging scenario, pooled sensitivity and specificity were 0.76 (0.74-0.78) and 0.45 (0.27-0.58), respectively, considering the identification of prostate cancer in each described situation. We also obtained specificity and sensitivity results for PSA subdivisions. Conclusion: 68Ga-PSMA PET provides higher sensitivity and specificity than traditional imaging for prostate cancer.


INTRODUCTION
Prostate cancer is the second most commonly diagnosed cancer in males worldwide and, in the United States, the most commonly diagnosed invasive cancer in males. Prostate cancer is also the fifth leading cancer-related cause of death worldwide (1). The increase in life expectancy will lead to an increase in disease incidence, which is becoming an epidemic in terms of male public health. In the United States, prostate cancer is also the second most common type of cancer and the second leading cause of cancer death (second only to lung cancer). In 2017, the incidence and deaths from this disease were 161.360 cases and 26.730 cases, respectively (2).
As the presence and location of the primary or recurrent tumors are critical for planning patient management, a vast range of imaging modalities are being assessed as tools for the evaluation of patients with prostate cancer in primary and secondary staging (1,2).
While the introduction of PSA-screening has led to earlier diagnosis of prostate cancer, a subset of patients developed high-risk of metastatic disease (3). A more accurate alternative for assessing disease staging is crucial for treatment decisions. However, all current conventional imaging modalities show limitations, and optimizing these imaging modalities is an intense and rapidly developing field of research. In the last decades, we have seen the development and improvement of functional imaging. Among those, combined positron emission tomography (PET)/computed tomography (CT) is one of the most promising techniques (4). sensitivity and specificity were 0.76 (0.74-0.78) and 0.45 (0.27-0.58), respectively, considering the identification of prostate cancer in each described situation. We also obtained specificity and sensitivity results for PSA subdivisions. Conclusion: 68 Ga-PSMA PET provides higher sensitivity and specificity than traditional imaging for prostate cancer. 68 Ga-PSMA PET-CT is a non-invasive diagnostic tool to evaluate prostate cancer with increased prostate-specific membrane antigen (PSMA) expression. PSMA is a protein expressed on dysplastic prostate cells, with levels of expression of 100-1000 times that of healthy cells, which increase even further with higher stages and grades of prostate cancer (5,6). 68 Ga is a positron emitter obtained from a 68 Ge/ 68 Ga generator system with 68min of half--life. PSMA demonstrated to have high affinity and specific internalization into prostate cancer cells (7). 68 Ga-PSMA-11 was first synthesized and evaluated by the Heidelberg group in Germany, and its accumulation is proportional to the expression level of PSMA (8). Other PSMA ligands, such as 68 Ga-PSMA-617 and 68 Ga-PSMA-I&T, showed similar distribution and image properties to 68 Ga-PSMA-11 (6,8), the reason why all three are known as 68 Ga-PSMA.
To date, the use of 68 Ga-PSMA has been well reported, and initial staging revealed superior sensitivity and specificity profiles compared to conventional choline-based tracers (7). Not only diagnosis but also among the management changes observed in the studies, the proportion of inter and intra-modality was relatively similar, indicating that 68 Ga-PSMA-PET may help better plan the optimal dose, site, and volume of radiation in the case of salvage radiotherapy. We systematically reviewed the literature outlining the use of 68 Ga--PSMA PET imaging in prostate cancer. Our primary objective was to perform a literature review to determine 68 Ga-PSMA PET accuracy in prostate cancer, and our secondary objective was to identify predictors of positive 68 Ga-PSMA PET.

Bibliographic search
A systematic review was performed under the Cochrane Collaboration and Preferred Reporting Items for Systematic Review and Meta-analysis (PRISMA) guidelines (9,10). We

Inclusion and exclusion criteria
Patients: patients diagnosed with prostate cancer and patients initially submitted to radical prostatectomy (RP) or radiotherapy with curative intent who showed biochemical recurrence defined as a prostate-specific antigen (PSA) elevation of ≥0.2ng/mL in patients with primary prostatectomy or as a PSA above the nadir after primary radiation therapy (according to the ASTRO 1996 criteria (11)).
Study design: Diagnostic accuracy cross--sectional study with prospective and retrospective approaches.
Exclusion criteria: case reports, animal, and phantom studies were excluded.
No language or sample-size restrictions were used.

Reference standard
A composite standard including changing in PSA values, clinical follow-up, and histopathological findings.

Outcome measures
The outcome measures included identification of predictors of 68 Ga-PSMA-PET positivity, sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and overall accuracy.

Study selection
The titles and abstracts retrieved from bibliographic searches were independently screened by two authors (MCS and PDB). The full texts of all relevant articles were obtained and independently assessed for inclusion by the same authors aforementioned. Studies that did not fulfill the inclusion criteria were excluded.

Quality assessment
The studies were independently assessed by two authors (MCS and PDB) using the Quality Assessment of Diagnostic Accuracy Studies-2 (QUADAS 2) checklist tool (12). The QUADAS-2 tool assesses four domains: risk of bias in patient selection, index test, reference standard, and the timing of reference standard. Each paper was scored independently by two evaluators (MCS and PDB), and discrepancies were discussed and resolved in common agreement.

Data extraction
The following information was extracted from each study: sample size, age of the patients, indication for PET (diagnosis, primary staging or recurrent disease staging), PSA level, previous therapies, initial cancer stage, 68 Ga-PSMA-11 PET characteristics, rates of positive PET, and pathology correlation data (when available). When pathology correlation data were available, numbers of true positives, false positives, true negatives and false negatives were collected as appropriated. The 68 Ga-PSMA-PET for diagnosis, primary staging, and recurrent cancer staging purposes data were displayed separately, when available.
The extracted data were collected in Excel 2007 (Microsoft Corporation, Redmond, CA, USA), and analysis was performed using Meta-Disc 1,4 (13). The detection rates were pooled using the generic inverse variance approach in the random--effects model (14). Heterogeneity in the meta--analysis of detection rates was assessed using the X 2 statistic in the I 2 statistic (10). The I 2 statistic indicates the percentage of the overall variability that can be attributed to between-study (or inter--study) variability, as opposed to within-study (or intra-study) variability. An I 2 more significant than 50% is considered to indicate substantial heterogeneity (10).
We explored the variability in diagnostic accuracy across studies by plotting the estimates of the observed sensitivities and specificities in forest plots and receiver-operating characteristic (ROC) space. Whenever data for computing true-positive, false-negative, true-negative, and false-positive rates were available, we performed meta-analyses using the bivariate model to produce summary sensitivities and specificities (13). We remade the calculations, excluding one article at a time, to verify if one of the studies was the main responsible for the heterogeneity in the meta-analyses. If substantial heterogeneity in the prevalence among the studies was observed, we estimated PPVs and NPVs by considering their relationship in the sensitivity, specificity and prevalence, and generating different scenarios with different prevalence. The significance level was set at α=0.05.
In patients undergoing a scan for disease recurrence, we correlated the data (sensitivity and specificity) to PSA level, in subgroups divided as follow: PSA level <0.5ng/mL, PSA level between 0.5ng/mL and 1ng/mL, PSA level between 1ng/mL and 2ng/mL and PSA level higher than 2ng/mL.

Pooled sensitivity, specificity, and diagnostic odds ratios; likelihood ratios
The availability of original data and the possibility of constructing a two-by-two table were minimum requirements (6, 15, 17, 18, 22-24, 32, 33, 36, 37, 39-49). We excluded studies that did not provide us with enough data in terms of sensitivity, specificity, diagnostic odds ratios, and likelihood ratios analyses to create a 2X2 Table. Overall, 34 studies met the criteria for this meta--analysis, comprehending a total of 4532 patients submitted to 68 Ga-PSMA PET; among those, nine studies (287 patients) in diagnostic setting and four studies for staging purposes, and 22 studies for restaging, in a total of 4050 patients (note that 1 study included staging and restaging patients).       (Figure-2). We carried out a secondary analysis, withdrawing studies, one by one, in order to identify which study generated such heterogeneity. Recalling Budaus, L 2015 study, we obtained a pooled sensitivity of 0.92 (0.89 to 0.95) with 2.6% heterogeneity (Figure-3).
The group of studies that enrolled patients for staging indication, resulted in a sensitivity of 0.93 (0.86-0.98), with a low inconsistency, and a specificity of 0.96 (0.92-0.99), but with a significant inconsistency (Figure-4). After a secondary analysis, when Herlermann, the study found to be responsible for the inconsistency, was withdrawn from the pool, the specificity was 0.99 (0.96-1.00) (Figure-5). A summary ROC (sROC) curve confirmed the high value for this imaging modality in the staging setting with an area under the curve of 0.9731 (Figure-6).
The pool of studies that analyzed the power of 68 Ga-PSMA PET in restaging recurrent prostate cancer resulted in a sensitivity of 0.76 (0.74 to 0.78), with heterogeneity of 96.7% (Figure-7). The specificity of the method, calculated based on the data available from the pool of studies, was 0.42 (0.27-0.58), and, again, a great inconsistency was noted (Figure-8). An sROC curve was plotted for those results, and the calculated area under the curve resulted in 0.73 (Figure-9).
When assessing biochemical recurrence, we observed that the higher the PSA level, the higher was the ability of the study to accurately demonstrate sites of accumulation of PSMA, positive for prostate cancer involvement. For patients with PSA LEVEL <0.5ng/mL, the positive LR pool estimation was 1.17 (0.37-3.73) and the sensitivity was 56% (0.42-0.68), increasing to 1.04 (0.38-    2.85) and 58% (0.47-0.68), respectively, in patients with PSA between 0.5 and 1ng/mL. The positive probable pool ratio is 1.44 (0.59-3.51) and the sensibility is 81% (0.74-0.87) for patients with PSA 1 to<2ng/mL and, for patients with a PSA level higher than 2ng/mL, LR ratio is 1.78 (0. 66-4.77), with sensitivity of 96% (0.93-0.97).

DISCUSSION
Early biochemical recurrence represents the most clinically relevant subgroup of patients with relapsed prostate cancer, offering the possibility of potential curative salvage therapy concepts that might have a major impact on the outcome of patients. In bioche-  mical recurrence, traditional imaging approaches (MRI, CT, and choline-based PET-CT) often fail to localize disease, mainly when the PSA level ranges lower than 2ng/mL. In our meta--analysis, we defined 4 subgroups based on PSA level: very low (<0.5ng/mL), low (0.5-1.0ng/mL), intermediate (1-2.0ng/mL), and high (>2.0ng/mL). We confirmed the value of 68 Ga-PSMA-PET in all subgroups. Detection rates are indeed higher when PSA levels are higher; however, the 68 Ga-PSMA--PET may have the greatest clinical impact in very low and low PSA level subgroups. To date, choline-based PET/CT is not recommended for patients with recurrent cancer and PSA level below 2ng/ mL. Compared to choline-based PET/CT, evidence suggests that the 68 Ga-PSMA PET has better sensitivity in detecting prostate cancer recurrence. On pooled analysis, the 68 Ga-PSMA PET sensitivity was 56% for PSA under 0.5ng/mL. Our results for diagnostic accuracy are according to a recently published meta-analysis, reporting similar pooled sensitivity and specificity (3). To the date, most of the data outlining the utility of the 68 Ga-PSMA PET is in the restaging for biochemical recurrence after definitive therapy.
Thus far, the value of 68 Ga-PSMA PET in the accurate detection and delineation of intraprostatic tumor burden, which is important for diagnosis and treatment planning for patients with primary prostate cancer, is poorly explored. Although prostate cancer is mostly a multifocal disease, there is growing evidence that dominant intraprostatic lesions (DILs) may be responsible for metastatic and recurrent prostate cancer. Most of the ongoing studies use multiparametric magnetic resonance imaging (mpMRI); however, Zamboglou et al. compared seven patients who underwent mpMRI and 68 Ga-PSMA PET before radical prostatectomy with co-registration between 68 Ga-PSMA PET, mpMRI and histopathology. The sensitivity and specificity for 68 Ga-PS-MA PET were 75% and 87% and for mpMRI were 70% and 82%, respectively.
The present study highlights the possibility of improvement in evaluating prostate cancer, supporting the use of 68 Ga-PSMA PET for diagnosing and staging. Traditionally, the primary staging of lymph nodes is performed using CT or MRI, but this relies on pathologic changes in lymph node morphology and size criteria. However, up to 80% of metastasis-involved nodes are smaller than the threshold limit of 8mm, typically used in clinical practice. Meta-analytical data for the traditional CT and MRI imaging approaches suggest sensitivity and specificity be 39-42% and 82%, respectively. A recent meta-analysis of choline-based tracers for PET-CT reviewed sensitivity of 49.2% and specificity of 95% for the detection of lymph nodes. Maurer et al. (34) performed a retrospective review of 130 patients undergoing high-risk prostate cancer, with a sensitivity of 65.9% and specificity of 98.9%. In our meta-analysis, the pool of studies showed the sensitivity and specificity were 93% and 87%, respectively, in detecting positive lymph nodes. As these are superior to those for traditional imaging, 68 Ga-PSMA PET could allow complete and accurate diagnosing in primary staging compared to the current practice and potential improvement in patient care. Despite significant advances, there is considerable scope for further research based on the use of 68 Ga-PSMA PET. There is a need for more robust sensitivity and specificity data. In this meta-analysis, much of the histopathological correlation data available were not suitable for inclusion in our analysis for biopsy was performed according to clinician discretion. Selective lesion biopsy did not provide meaningful false-negative rates or specificity. Several groups reported the use of the 68 Ga-PSMA PET for localization of intraprostatic malignancies, particularly in the context of focal therapies (50).
There are several limitations to this study. Firstly, most of the articles used for meta-analysis were derived from not measurable, retrospective, and single-institutional studies. The absence of more comprehensive studies could be justified as this technique is new and is still being explored for its inclusion in recent prostate cancer guidelines. Secondly, the heterogeneous nature of patient cohorts, treatment protocols, and studies used to pool sensitivity and specificity data, in particular, because most of the studies were carried with small sample size and patients were assessed in primary staging settings. Researchers should be encouraged to agree on data acquisition protocols and data analysis algorithms to increase the comparability of studies, which is one of the major issues in achieving evidence. Careful reporting of those methods may help evaluation to the extent that the results found to be applicable in other clinical settings. 68 Ga-PSMA PET seems to provide higher sensitivity and specificity compared to alternative techniques. Our results reinforce the current evidence of the usefulness of 68 Ga-PSMA PET, whereby the diagnostic evidence is more substantial in restaging with biochemical recurrence. The main applicability of 68 Ga-PSMA PET certainly relies on restaging patients with biochemical recurrence after local treatment with curative intent.